In a low density parity check (LDPC) coding scheme, it is conventionally known that a high decoding gain is obtained and thus decoding characteristics extremely close to the Shannon limit are obtained by performing iterative decoding on an encoded data signal by using an SP (sum-product) algorithm or the like.
In the low density parity check (LDPC) coding scheme, it is known that, although decoding performance for random errors is extremely good, decoding performance for continuous (burst) errors (burst errors) is extremely bad. In other words, a conventional LDPC coding and iterative decoding system has a problem in that iterative decoding performed during the process of decoding an LDPC code causes the propagation (dispersion) of erroneous data that occurs during a burst error interval and thus decoding performance (error correction performance) is significantly decreased.
Therefore, there is a need for a method for detecting and specifying a burst error interval in some way and performing decoding that is resistant to burst errors, thereby suppressing the propagation (dispersion) of errors during the process of decoding. It should be noted that burst errors means errors that are continuous over long-interval data. Depending on the generation mechanism, in a magnetic recording and reproducing system such as a hard disk drive, there are burst errors such as a medium deficit, a thermal asperity caused by a head crash, or a pole-erase phenomenon caused by overwriting from an adjacent track due to residual magnetism.
Patent Document 1 discloses a data storage device that determines whether log likelihood ratio (LLR) information output from a channel decoder that is an iterative decoder that performs an iterative decoding process on encoded data obtained by further adding an RLL (Run-Length Limited) code violates the RLL code side in order to detect a burst error interval and that attenuates a log likelihood ratio corresponding to the detected burst error interval in order to perform adjustment.
Moreover, Patent Document 2 discloses a recording and reproducing device that detects the generation of a thermal asperity in response to the fluctuation of signal amplitude, outputs an erasure flag signal, and masks channel information to perform iterative calculations over the time period for which the erasure flag is on, thereby not requiring an ECC (Error Correcting Code).
Moreover, Patent Document 3 discloses a decoder that determines whether each data of each block is true or not based on hard information according to an ECC code with respect to encoded data obtained by further adding the ECC code to the encoded data and that replaces the likelihood of each data of a block for which the determination value is true (without error) with a maximum value.
Moreover, Non-Patent Documents 1 and 2 disclose a decoding method for detecting the degradation of regeneration amplitude in order to specify a burst error interval and for performing a manipulation by which the LLR of the burst error interval is set to zero in order to suppress the propagation of errors during the process of iterative decoding. For this decoding method, it is assumed that a medium defect causes the burst errors.    [Patent Document 1] Japanese Patent Application Laid-open No. 2005-166089    [Patent Document 2] Japanese Patent Application Laid-open No. 2003-68024    [Patent Document 3] Japanese Patent Application Laid-open No. 2008-65969    [Non-Patent Document 1] Yasuaki Nakamura et al., “A Study on LDPC Coding and Iterative Decoding System using Burst Information”, Dec. 8, 2005, IEICE technical report. Magnetic recording, Vol. 105, No. 473, pp. 1 to 6    [Non-Patent Document 2] Yasuaki Nakamura et al., “Performance Comparison of Error Correction Code in Perpendicular Magnetic Recording and Reproducing Systems with Burst”, May 31, 2007, IEICE technical report. Magnetic recording, Vol. 107, No. 84, pp. 55 to 60